Italian

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Coagulation is a complex process by which
blood forms clots. It is an important part of
hemostasis (the
cessation of blood loss from a damaged vessel) whereby a damaged
blood
vessel wall is covered by a platelet and fibrin containing clot to stop
bleeding and begin
repair of the damaged vessel. Disorders of coagulation can lead to
an increased risk of bleeding (hemorrhage) and/or clotting
(thrombosis).

Coagulation is highly conserved
throughout biology; in all mammals, coagulation involves
both a cellular (platelet) and a protein (coagulation factor)
component. The system in humans has been the most extensively
researched and therefore is the best understood.

Coagulation is initiated almost instantly after
an injury to the blood vessel damages the endothelium (lining of the
vessel). Platelets
immediately form a hemostatic plug at the site of injury; this is
called primary hemostasis. Secondary hemostasis occurs
simultaneously; proteins
in the blood
plasma, called coagulation factors, respond in a complex
cascade to form fibrin
strands which strengthen the platelet plug.

The coagulation cascade

The coagulation cascade of
secondary hemostasis has two pathways, the contact activation
pathway (formerly known as the intrinsic pathway) and the tissue
factor pathway (formerly known as the extrinsic pathway) that lead
to fibrin formation. It was previously thought that the coagulation
cascade consisted of two pathways of equal importance joined to a
common pathway. It is now known that the primary pathway for the
initiation of blood coagulation is the tissue factor pathway. The
pathways are a series of reactions, in which a zymogen (inactive enzyme
precursor) of a serine
protease and its glycoprotein co-factor are
activated to become active components that then catalyze the next
reaction in the cascade, ultimately resulting in cross-linked
fibrin. Coagulation factors are generally indicated by Roman
numerals, with a lowercase a appended to indicate an active
form.

The coagulation factors are generally serine
proteases (enzymes).
There are some exceptions. For example, FVIII and FV are
glycoproteins and Factor XIII is a transglutaminase.
Serine proteases act by cleaving other proteins at specific sites.
The coagulation factors circulate as inactive zymogens.

The coagulation cascade is classically divided
into three pathways. The tissue factor and contact activation
pathways both activate the "final common pathway" of factor X,
thrombin and fibrin.

Tissue factor pathway

The main role of the tissue factor
pathway is to generate a "thrombin burst", a process by which
thrombin, the most
important constituent of the coagulation cascade in terms of its
feedback activation roles, is released instantaneously. FVIIa
circulates in a higher amount than any other activated coagulation
factor.

Following damage to the blood vessel, endothelium Tissue Factor
(TF) is released, forming a complex with FVII and in so doing,
activating it (TF-FVIIa).

Thrombin then activates other components of the coagulation
cascade, including FV and FVIII (which activates FXI, which in turn
activates FIX), and activates and releases FVIII from being bound
to vWF.

FVIIIa is the co-factor of FIXa and together they form the
"tenase" complex which
activates FX and so the cycle continues. ("Tenase" is a contraction
of "ten" and the suffix "-ase" used for enzymes.)

Contact activation pathway

The contact activation pathway
begins with formation of the primary complex on collagen by
high-molecular weight kininogen (HMWK), prekallikrein, and FXII
(Hageman factor). Prekallikrein
is converted to kallikrein and FXII becomes
FXIIa. FXIIa converts FXI into FXIa. Factor XIa activates FIX,
which with its co-factor FVIIIa form the tenase complex, which activates
FX to FXa. The minor role that the contact activation pathway has
in initiating clot
formation can be illustrated by the fact that patients with
severe deficiencies of FXII, HMWK, and prekallikrein do not have
a bleeding disorder.

Final common pathway

Thrombin has a large array of
functions. Its primary role is the conversion of fibrinogen to fibrin, the
building block of a hemostatic plug. In addition, it activates
Factors VIII and V and their inhibitor protein C (in
the presence of thrombomodulin), and it
activates Factor XIII, which forms covalent
bonds that crosslink the fibrin polymers that form from
activated monomers.

Following activation by the contact factor or
tissue factor pathways the coagulation cascade is maintained in a
prothrombotic state by the continued activation of FVIII and FIX to
form the tenase complex,
until it is down-regulated by the anticoagulant pathways.

Cofactors

Various substances are required for the proper
functioning of the coagulation cascade:

Calcium
and phospholipid (a
platelet membrane
constituent) are required for the tenase and prothrombinase
complexes to function. Calcium mediates the binding of the
complexes via the terminal gamma-carboxy residues on FXa and FIXa
to the phospholipid surfaces expressed by platelets as well as
procoagulant microparticles or microvesicles shedded from
them. Calcium is also required at other points in the coagulation
cascade.

Vitamin
K is an essential factor to a hepatic gamma-glutamyl
carboxylase that adds a carboxyl group to glutamic
acid residues on factors II, VII, IX and X, as well as Protein S,
Protein C and Protein Z. In
adding the gamma-carboxyl group to glutamate residues on the
immature clotting factors Vitamin K is itself oxidized. Another
enzyme,
Vitamin K epoxide reductase, (VKORC) reduces vitamin K back to
its active form. Vitamin K epoxide reductase is pharmacologically
important as a target for anticoagulant drugs warfarin and related coumarins such as acenocoumarol, phenprocoumon and dicumarol. These drugs create
a deficiency of reduced vitamin K by blocking VKORC, thereby
inhibiting maturation of clotting factors. Other deficiencies of
vitamin K (e.g. in malabsorption), or disease
(hepatocellular
carcinoma) impairs the function of the enzyme and leads to the
formation of PIVKAs (proteins formed in vitamin K absence) this
causes partial or non gamma carboxylation and affects the
coagulation factors ability to bind to expressed
phospholipid.

Regulators

Five mechanisms keep platelet activation and the
coagulation cascade in check. Abnormalities can lead to an
increased tendency toward thrombosis:

Protein
C is a major physiological anticoagulant. It is a vitamin
K-dependent serine protease enzyme that is activated by thrombin
into activated protein C (APC). Protein C is activated in a
sequence that starts with Protein C and thrombin binding to a cell
surface protein thrombomodulin.
Thrombomodulin binds these proteins in such a way that it activates
Protein C. The activated form, along with protein S and a
phospholipid as cofactors, degrades FVa and FVIIIa. Quantitative or
qualitative deficiency of either may lead to thrombophilia (a tendency
to develop thrombosis). Impaired action of Protein C (activated
Protein C resistance), for example by having the
"Leiden" variant of Factor V or high levels of FVIII also may
lead to a thrombotic tendency.

Antithrombin
is a serine
protease inhibitor (serpin) that degrades the serine
proteases; thrombin, FIXa, FXa, FXIa and FXIIa. It is constantly
active, but its adhesion to these factors is increased by the
presence of heparan
sulfate (a glycosaminoglycan)
or the administration of heparins (different heparinoids
increase affinity to FXa, thrombin, or both). Quantitative or
qualitative deficiency of antithrombin (inborn or acquired, e.g. in
proteinuria) leads
to thrombophilia.

Plasmin
is generated by proteolytic cleavage of plasminogen, a plasma
protein synthesized in the liver. This cleavage is catalyzed by
tissue plasminogen activator (t-PA) which is synthesized and
secreted by endothelium. Plasmin proteolytically cleaves fibrin
into fibrin degradation products which inhibits excessive fibrin
formation.

Prostacyclin
(PGI2) is released by endothelium and activates platelet Gs protein
linked receptors. This in turn activates adenylyl
cyclase which synthesizes cAMP. cAMP inhibits platelet
activation by counteracting the actions that result from increased
cytosolic levels of calcium and by doing so inhibits the release of
granules that would lead to activation of additional platelets and
the coagulation cascade.

Fibrinolysis

Eventually, all blood clots are reorganised
and resorbed by a process termed fibrinolysis. The main
enzyme responsible for this process (plasmin) is regulated by various
activators and inhibitors.

Testing of coagulation

Numerous tests are used to assess
the function of the coagulation system:

The contact factor pathway is initiated by
activation of the "contact factors" of plasma, and can be measured
by the
activated partial thromboplastin time (aPTT) test.

The tissue factor pathway is initiated by release
of tissue
factor (a specific cellular lipoprotein), and can be measured
by the prothrombin
time (PT) test. PT results are often reported as ratio
(INR
value) to monitor dosing of oral anticoagulants such as warfarin.

The quantitative and qualitative screening of
fibrinogen is measured by the thrombin
clotting time (TCT). Measurement of the exact amount of
fibrinogen present in the blood is generally done using the
Clauss
method for fibrinogen testing. Many analysers are capable of
measuring a "derived fibrinogen" level from the graph of the
Prothrombin time clot.

If a coagulation factor is part of the contact or
tissue factor pathway, a deficiency of that factor will affect only
one of the tests: thus hemophilia
A, a deficiency of factor VIII, which is part of the contact
factor pathway, results in an abnormally prolonged aPTT test but a
normal PT test. The exceptions are prothrombin, fibrinogen and some
variants of FX which can only be detected by either aPTT or PT. If
an abnormal PT or aPTT is present additional testing will occur to
determine which (if any) factor is present as aberrant
concentrations.

Deficiencies of fibrinogen (quantitative or
qualitative) will affect all screening tests.

Role in disease

Problems with coagulation may dispose to
hemorrhage, thrombosis, and occasionally both, depending on the
nature of the pathology.

Disease and clinical significance of thrombosis

The
best-known coagulation factor disorders are the hemophilias. The three main
forms are hemophilia A
(factor VIII deficiency), hemophilia B
(factor IX deficiency or "Christmas disease") and hemophilia C
(factor XI deficiency, mild bleeding tendency). Hemophilia A and B
are X-linked recessive disorders whereas Hemophilia C is much more
rare autosomal dominant disorder most commonly seen in Ashkenazi
Jews.

von
Willebrand disease (which behaves more like a platelet disorder
except in severe cases), is the most common hereditary bleeding
disorder and is characterized as being inherited autosomal
recessive or dominant. In this disease there is a defect in von
Willebrand factor (vWF) which mediates the binding of glycoprotein
Ib (GPIb) to collagen. This binding helps mediate the activation of
platelets and formation of primary hemostasis.

Bernard-Soulier syndrome there is a defect or
deficiency in GPIb. GPIb, the receptor for vWF, can be defective
and lead to lack of primary clot formation (primary hemostasis) and
increased bleeding tendency. This is an autosomal recessive
inherited disorder.

Thrombasthenia of Glanzman and Naegeli (Glanzmann
thrombasthenia) is extremely rare. It is characterized by a
defect in GPIIb/IIIa fibrinogen receptor complex. When GPIIb/IIIa
receptor is dysfunctional fibrinogen cannot cross-link platelets
which inhibits primary hemostasis. This is an autosomal recessive
inherited disorder. In liver
failure (acute and chronic forms) there is insufficient
production of coagulation factors by the liver; this may increase
bleeding risk.

Deficiency of Vitamin K may also contribute to
bleeding disorders because clotting factor maturation depends on
Vitamin K.

Thrombosis is
the pathological development of blood clots. These clots may break
free and become mobile forming an embolus or grow to such a size
that occludes the vessel in which it developed. An embolism is said to occur when
the thrombus (blood
clot) becomes a mobile embolus and migrates to another part of the
body, interfering with blood circulation and hence impairing organ
function downstream of the occlusion. This causes ischemia and often leasds to
ischemic necrosis of
tissue. Most cases of thrombosis are due to acquired extrinsic
problems (surgery,
cancer, immobility, obesity, economy
class syndrome), but a small proportion of people harbor
predisposing conditions known collectively as thrombophilia (e.g.
antiphospholipid
syndrome, factor V
Leiden and various other rarer genetic disorders).

Mutations in factor XII
have been associated with an asymptomatic prolongation in the
clotting time and possibly a tendency towards thrombophlebitis. Other
mutations have been linked with a rare form of hereditary angioedema (type
III).

Pharmacology

Procoagulants

The use of adsorbent chemicals, such as
zeolites, and other
hemostatic
agents is also being explored for use in sealing severe
injuries quickly. Thrombin and fibrin glue are used surgically to
treat bleeding and to thrombose aneurysms.

Tranexamic
acid and aminocaproic
acid inhibit fibrinolysis, and lead to a de facto reduced
bleeding rate. Before its withdrawal, aprotinin was used in some
forms of major surgery to decrease bleeding risk and need for blood
products.

Of the anticoagulants, warfarin (and related coumarins) and heparin are the most commonly
used. Warfarin interacts with vitamin K, while heparin and related
compounds increase the action of antithrombin on thrombin and
factor Xa. A newer class of drugs, the direct
thrombin inhibitors, is under development; some members are
already in clinical use (such as lepirudin). Also under
development are other small molecular compounds that interfere
directly with the enzymatic action of particular coagulation
factors (e.g. rivaroxaban).

Coagulation factors

History

Initial discoveries

Theories on the coagulation of blood
have existed since antiquity. Physiologist Johannes
Müller (1801-1858) described fibrin, the substance of a
thrombus. Its soluble precursor, fibrinogen, was thus named by
Rudolf
Virchow (1821-1902), and isolated chemically by Prosper
Sylvain Denis (1799-1863).
Alexander Schmidt suggested that the conversion from fibrinogen
to fibrin was the result of an enzymatic process, and labeled
the hypothetical enzyme "thrombin" and its precursor "prothrombin".
Arthus
discovered in 1890 that calcium was essential in coagulation.
Platelets
were identified in 1865, and their function was elucidated by
Giulio
Bizzozero in 1882.

The theory that thrombin was generated by the
presence of tissue
factor was consolidated by Paul
Morawitz in 1905. At this stage, it was known that
thrombokinase/thromboplastin (factor III) was released by damaged
tissues, reacting with prothrombin (II), which, together with
calcium
(IV), formed thrombin, which converted fibrinogen into fibrin
(I).

Coagulation factors

The remainder of the biochemical
factors in the process of coagulation were largely discovered in
the 20th
century.

A first clue as to the actual complexity of the
system of coagulation was the discovery of proaccelerin (initially
and later called Factor V) by Paul Owren (1905-1990) in 1947. He
also postulated that its function was the generation of accelerin
(Factor VI), which later turned out to be the activated form of V
(or Va); hence, VI is not now in active use. in the UK and by Davie
and Ratnoff in the USA, respectively.

Nomenclature

The usage of Roman
numerals rather than eponyms or systematic names was agreed
upon during annual conferences (starting in 1955) of hemostasis
experts. In 1962, consensus was achieved on the numbering of
factors I-XII. This committee evolved into the present-day
International Committee on Thrombosis and Hemostasis (ICTH).
Assignment of numerals ceased in 1963 after the naming of Factor
XIII. The names Fletcher Factor and Fitzgerald Factor were given to
further coagulation-related proteins, namely prekallikrein and
high molecular weight kininogen respectively.

Factors III and VI are unassigned, as
thromboplastin was never identified, and actually turned out to
consist of ten further factors, and accelerin was found to be
activated Factor V.

Other species

All mammals have an extremely closely related
blood coagulation process, using a combined cellular and serine
protease process. In fact, it is possible for any mammalian
coagulation factor to "cleave" its equivalent target in any other
mammal. The only nonmammalian animal known to use serine proteases
for blood coagulation is the horseshoe
crab.